1. Field of the Disclosure
This invention relates to a calcium phosphate bone graft material, to a process for making the calcium phosphate bone graft material, and to an osteoimplant fabricated from the calcium phosphate bone graft material. More particularly, this invention relates to a calcium phosphate bone graft material composed of plasma sprayed calcium phosphate wherein the relative amounts of amorphous (glassy) phase and crystalline (ceramic) phase varies from about 100 volume % amorphous, 0 volume % crystalline to about 30 volume % amorphous, 70 volume % crystalline.
2. Description of the Related Art
An ideal artificial bone graft possesses an osteoconductive surface that bonds to bone, and dissolves (resorbs) at about the rate of bone growth so that the formation of new bone is not inhibited. Existing artificial bone grafting materials are usually quite osteoconductive, but most resorb either too quickly or too slowly.
The most commonly employed artificial grafting materials are ceramic forms of hydroxylapatite (HA), or of hydroxylapatite/tricalcium phosphate (TCP) mixtures. These ceramic materials are obtained by sintering HA or HA/TCP. Sintering is a heating process whereby crystals grow larger and more perfect to the limit where all crystals completely surround other crystals and all porosity is eliminated. The ceramic structure is therefore characterized by well-defined crystals held together by grain boundaries where different crystals touch. The more perfect the sintering conditions are, the more perfectly formed the crystals are, and the more crystalline the material is. Conventionally employed HA or HA/TCP artificial bone grafting materials contain a minimum of 80 volume % crystallinity with amounts of up to 100 volume % not being uncommon.
These highly crystalline calcium phosphate ceramics are reasonably osteoconductive (i.e., they bond to and support the growth of bone), but they resorb much more slowly than the rate of bone growth since more energy is required to disrupt a perfect crystal than a disordered structure. One approach to improving the resorbability of calcium phosphate grafting materials is to partially sinter the ceramic so that the crystals are less perfect. However, there are limits to this approach because strength decreases as the amount of sintering is decreased.
In another approach to make the ceramic calcium phosphates more resorbable, the calcium to phosphorous ratio is lowered from the 1.67 of HA to the 1.5 of TCP (or somewhere in between for mixtures of the two), to take advantage of the slightly greater solubility of TCP. TCP is more soluble than HA because only HA is stable in the presence of water (TCP is metastable). Thus, there is a driving force for TCP to dissolve since it is not stable in water. Once dissolved, it can remain in solution (if conditions are favorable), or it can later precipitate as HA by picking up additional calcium from the solution. The disadvantage to this approach is that bone is essentially HA, and TCP does not have as good bone bonding properties as HA. Other artificial bone graft materials that are not calcium phosphate ceramics include calcium sulfate, calcium carbonate, and bioglasses. Compared to the ceramics, these materials resorb much faster (sometimes faster than the rate of bone growth) and sometimes exhibit better bone bonding ability (especially the bioglasses). Bioglasses are based on short chains of silicon dioxide with added calcium and phosphate. Upon exposure to water, the calcium and phosphate reprecipitate on the surface, forming a biological HA type material. Bone bonding is excellent due to the biological apatite surface, and resorption is at about the rate of bone growth, or faster (depending on the application). Also known are glassy forms of calcium phosphates made by melting the phosphates in the presence of metals such as iron, lithium, extra calcium, etc. These glassy forms of calcium phosphates exhibit many of the properties of bioglasses, but usually have even faster resorption rates. These materials are essentially 100% glassy.
What can learned from the above is that calcium phosphate glasses possess good bone grafting properties, so long as the resorption rate is not too fast. A glass is a much more disordered structure than a ceramic or crystal, and, like all non-HA forms of calcium phosphate, is not stable in the presence of water. These properties mean that calcium phosphate glasses possess significantly higher dissolution rates than calcium phosphate ceramics, and the problem is keeping the resorption rates under control.
The bone bonding of calcium phosphate glasses is good because the relatively high solubility of the calcium and phosphate encourages the dissolution of material from the glass surface followed by reprecipitation as “bone” apatite on the surface. Bone apatite is an impure form of HA that incorporates many additional materials from surrounding body fluids such as carbonate (substituting for OH groups and/or phosphate groups), and metals such as manganese that can replace calcium.
To date, the most widely accepted way of obtaining a calcium phosphate glassy material for bone grafting is to produce a bioglass which, as mentioned above, has a silica base. A glass is made by heating a material until it melts, then cooling it in such a way that the melted, disordered structure is largely preserved. This is a slow process requiring hours or days of heating, followed by slow or fast cooling.
For many years ceramic powders made of HA (and HA/TCP or TCP) have been plasma sprayed on to orthopedic or dental implants to form osteoconductive coatings, thereby improving the integration of the implants into bone. The ceramic powder is injected into a plasma flame at temperatures of 10 to 15 times the decomposition temperatures of the ceramic, but remains in the plasma flame for only a few milliseconds while it travels to the surface of the object to be coated, where the powder immediately cools as it gives its heat up to the much larger object. When calcium phosphate ceramic powders are plasma sprayed on to implants, a two-phase coating is formed, consisting of a melted phase and an unmelted phase comprising unmelted particle cores. The melted phase yields no X-ray diffraction (XRD) pattern and therefore is non-crystalline. The unmelted particle core yields an XRD pattern that is little changed from that of the ceramic powder. The preservation of the initial ceramic powder is, in part, a consequence of the extreme rapidity of the plasma spray process.
The generally accepted theory behind plasma sprayed coatings is that the crystalline, ceramic phase is the functional part of the plasma sprayed calcium phosphate coating. Bone bonds to this ceramic phase in the coating the same as it bonds to solid, ceramic implants. However, the opposite is actually true. The amorphous, non-crystalline phase is responsible for the excellent biological properties of calcium phosphate coatings. Plasma sprayed coatings exhibit a better bone response than the corresponding non-plasma sprayed solid calcium phosphate ceramics, and also resorb at a faster rate. In their osteoconductive and resorption properties, plasma sprayed coatings can be considered to be between calcium phosphate ceramics and bioglasses.
The amorphous phase is a type of calcium phosphate glass formed by extremely rapid melting and extremely rapid cooling. Glassy materials can be produced by any method that provides very fast heating and cooling. For example, laser melting can be adapted to this process, as well as all forms of plasma spraying including flame spraying, vacuum plasma spraying, high velocity oxygen fuel (HVOF) spraying, etc. For purposes of this invention, plasma spraying shall be understood as being inclusive of all methods that provide very fast heating and cooling.
The improved biological response of plasma sprayed HA or other calcium phosphates compared to ceramic forms render them desirable as bone grafting materials. However, plasma sprayed calcium phosphates are currently only available as coatings, and not as grafting materials in their own right.